The invention relates to an exhaust-gas turbine.
German Published Patent Application No. 196 45 388 describes an exhaust-gas turbine, in which a guide-blade cascade can be engaged axially into an annular space between a spiral inlet duct and a rotor. The guide-blade cascade has, on its free end face, a cover disc which, when the guide-blade cascade is in a pushed-back position, covers an annular gap between the turbine casing and an inner part for guiding the guide-blade cascade. The guide-blade cascade and the inner part are exposed to the high temperatures of the exhaust gases. They also experience pronounced temperature fluctuations in the different operating regions, thus leading to thermal stresses and different thermal expansions. The latter may cause distortions of the components, as a result of which, on the one hand, the geometric designs may change and, on the other hand, frictional losses may occur in guides on the moved components. Moreover, the actuating device and the guide-blade cascade may be jammed in the guides. Furthermore, the components are exposed to the corrosive influence of the aggressive exhaust gases.
It is an object of the present invention to provide an exhaust-gas turbine in which the guide-blade cascade with the axial slide, the inner part with its guide an/or a sliding sleeve is formed of a material that can withstand the operating conditions and which is easily machinable.
The above and other beneficial objects of the present invention are achieved by providing an exhaust-gas turbine as described herein. According to one embodiment of the present invention, the guide-blade cascade with the axial slide, the inner part and/or the sliding sleeve are produced from an austenitic cast steel material having the following components:
C less than 0.2%;
Si 1.5-2.5%;
Mnxe2x89xa62.00%;
Pxe2x89xa60.045%;
Sxe2x89xa60.03%;
Cr 24.0-26.0%;
Ni 19-22%; and
Wxe2x89xa65%.
This material provides high wear resistance, thermal resistance and corrosion resistance and is therefore suitable for use in a harsh environment. It is weldable and may be machined mechanically in a simple manner, with the result that the components are easy to produce. Due to the low coefficient of thermal expansion of the material, the components change their shape only slightly under the effect of the heat of the exhaust gases. The risk that a moved component will be jammed in its guide is therefore extremely low.
According to the present invention, the fraction of tungsten may be below 5%, such as, for example, between 1.4% and 1.8% or approximately 2%. In addition to tungsten, niobium, tantalum and/or molybdenum may be alloyed approximately in the same order of magnitude, and care should be taken to ensure that the formation of xcex4-ferrite is avoided.
Components made of the material according to the present invention are distinguished by fault-free long-term use under high temperatures and temperature fluctuations.